1. Field of the Invention
The present invention relates to a control valve driven by a stepping motor, and in particular, to a control valve suitable for an automotive air conditioner.
2. Description of the Related Art
In a vehicle in which an internal-combustion engine is mounted, it recently becomes difficult that the temperature of the cooling water in the vehicle, which has been used as a heat source, is raised to the temperature necessary for heating the interior of the vehicle, partly because the combustion efficiency of the engine has been increased. On the other hand, in a hybrid vehicle in which an internal-combustion engine and an electric motor are used in combination, it is further difficult to use cooling water for such the purpose, because the working ratio of the internal-combustion engine is low. In an electric car, there is no heat source itself created by an internal-combustion engine. Accordingly, an automotive air conditioner driven by a heat pump is presented, which can dehumidify and heat the interior of a vehicle by performing a cycle operation in which a refrigerant is used for not only cooling but also heating.
Such an automotive air conditioner has a refrigeration cycle including a compressor, an external heat exchanger, an evaporator, and an internal heat exchanger, etc., and the function of the external heat exchanger is switched between a heating operation and a cooling operation. During the heating operation, the external heat exchanger functions as an evaporator. In the case, the internal heat exchanger radiates heat while a refrigerant is circulating the refrigeration cycle, the heat heating the air in the interior of a vehicle. On the other hand, during the cooling operation, the external heat exchanger functions as a condenser. In the case, the refrigerant condensed in the external heat exchanger evaporates in the evaporator, and latent heat of evaporation thus generated cools the air therein. At the time, the air is also dehumidified.
When a plurality of evaporators function in accordance with an operating state of a refrigeration cycle in this way, it is needed to adjust a ratio of a flow rate of a refrigerant flowing through each evaporator. The same is true with the case where a plurality of condensers function. Accordingly, a control valve, by which an opening degree of a valve can be electrically adjusted, is sometimes provided at a specific position in a refrigerant circulation passage; in general, however, an electromagnetic valve driven by a solenoid, by which a large drive force can be obtained at a relatively low cost, is often used. When it is needed to precisely control an opening degree of a valve, however, it is desirable to use a control valve driven by a stepping motor, as frequently seen in residential air conditioners (see, for example, Patent Documents 1 and 2). It is because a displacement amount of a valve element, and eventually an opening degree of a valve can be precisely adjusted by setting the number of steps (number of drive pluses).